SCR Power Control. The Watlow Educational Series Book Six

SCR Power Control The Watlow Educational Series Book Six SCR Power Control Contents Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . ....
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SCR Power Control The Watlow Educational Series Book Six

SCR Power Control Contents Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 SCR Power Control in the Thermal System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 Silicone Controlled Rectifier (SCR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 SCR - A Modified Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Triac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7 SCR Firing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Electrical Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Zero-Cross Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Contactor Firing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 Burst Firing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 Phase Angle Firing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 SCR Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 Power Control and Heater Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 Electromechanical Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 Mercury Displacement Relay (MDR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 Solid State Relay (SSR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 Burst Firing SCR Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 Phase Angle Firing SCR Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 Power Control Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17 Stable Resistance Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17 Variable Resistance Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18 Phase Angle Soft Start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18 Phase Angle Current Limiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19 Transformer-Coupled Heating Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20 SCR Selection Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20 3 Phase SCR Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22 Booklet Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23 Answers to Exercises and Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . .25

© 1995 Watlow Electric Manufacturing Company. This document is protected under U.S. copyright law. Any duplication, other than by Watlow employees, without the express written consent of the publisher is forbidden.

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Watlow Educational Series Objectives The objectives of this booklet are to enable you to: • Name at least 2 reasons for using SCR power controllers. • Define what a SCR is and how it is used to allow AC to flow in both directions. • Define zero-cross switching and explain its benefit • Name the 2 types of burst firing and explain the effect time base has on them. • Explain phase angle firing and describe when it should be used. • Name and explain the 2 features provided with many phase angle fired power controllers. • Explain the effects of using a SCR power controller on heater life. • Describe which SCR firing methods should be used for which type of heater. • Identify the proper SCR configuration for 3 phase heaters.

Introduction A SCR power controller is an output device used for fast heater switching, to switch higher amperage electric heaters, to control variable resistance heaters and transformers coupled to resistance heaters. The many places where SCRs can be applied gives you many possibilities to successfully provide unique customer solutions! This book provides a thorough introduction to SCR power control principles. We first discover how a power controller fits into the thermal system. We then explore SCRs themselves, examine the firing methods used in SCRs and then look at how a power control affects heater life. From there, we journey through the application of SCRs for switching various heating loads. Finally, we give you the recommended SCR 3 phase configurations for various applications.

SCR Power Control in the Thermal System

The thermal system has a work load, heater, temperature sensor and temperature controller (Figure 1). As you learned in Book 5 (Temperature Control), the temperature controller output signals an output device to switch electric current ON and OFF to the heater. The output device is usually built into the controller case if possible (and that is what Figure 1 illustrates). The ON and OFF switching causes the resistance element inside of a heater to go through a continuous heating and cooling cycle (Figure 2). We know from Book 2 (Thermal Systems and Electric Heaters), that large temperature swings accelerate the oxidation of the resistance element in a heater. This is bad, since it reduces heater life.

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SCR Power Control SCR Power Control in the Thermal System

Figure 1 A Typical Thermal System

Temperature cycling accelerates oxidation by repeatedly cracking the oxide coating on a resistance element. Cracking and breaking off the protective oxide layer exposes fresh metal to oxygen attack.

Sensor Temperature Controlling Device

Work Load Heat Transfer Medium

Sensor Input

Heat Source

Output

Temperature

Figure 2 Resistance Element Temperature Swings Due to Slow ON/OFF Cycling

Time

Firing Gate

How can we reduce or eliminate the temperature swings of the heater's resistance element? We use a SCR power controller in the thermal system! A SCR power controller is made up of 3 distinct parts: the SCR (silicon controlled rectifier), sophisticated electronics which switch the SCR ON and OFF and a heat sink to dissipate the heat a SCR produces. A SCR is illustrated in Figure 3a. Examples of SCR power controllers are shown in Figures 3b and 3c. Figure 3 Firing Gate (Ring) Long Cycle Time

Short Cycle Time

Process Input Phase Angle or Burst Fired

1

⁄2"

a. A 75 amp SCR “Chip”

4

b. Watlow Power Series Controller Long Cycle Time

c. Watlow DIN-a-mite Power Control Short Cycle Time

Process Input Phase Angle or Burst Fired

Watlow Educational Series A SCR power controller is connected into the thermal system in Figure 4 below. Figure 4 The Thermal System with a SCR Power Controller

Sensor

Power Supply

Process Output

Temperature Controlling Device SCR Power Controller

Sensor Input

Work Load Heat Transfer Medium Heat Source

SCR Output

What has changed? The temperature controller output is now connected to the SCR power controller, not the heater. Notice that the current flowing to the heater passes through the power controller. The SCR power controller can now control the amount of electric current supplied from the power supply to the heater (called load current). How does the SCR control load current to the heater? First, the SCR power controller receives a process output signal from a temperature controller. The electronics use the signal to calculate how often the SCR must switch the heater ON and OFF. The electronics then adjust load current by very rapidly switching the SCR ON and OFF. This switching is often so fast, that the resistance element experiences very little temperature fluctuation (Figure 5). Figure 5 Reduced Element Temperature Swings Due to Very Fast ON/OFF Switching

Temperature

SCR Power Control in the Thermal System (con’t)

Time

Process Input Phase Angle or Burst Fired

5

SCR Power Control SCR Power Control in the Thermal System (con’t)

Because the temperature swings are greatly reduced or even eliminated, heater life increases dramatically! That is one of the major benefits of using SCR power controllers. Especially for large, expensive heaters, the SCR allows the customer to extend heater life as long as possible. Of course, the next question (if you haven't asked it already) is: What is a SCR and how does is control electric current to the heater? Excellent question! We address this next

Silicon Controlled Rectifier (SCR)

The silicon controlled rectifier (SCR) is a semiconductor. Another name for a SCR is a thyristor. Semiconductors, you may remember, are materials which are usually electric insulators. However, under certain conditions, they become electrical conductors. The most basic type of semiconductor is the diode. Diodes are occasionally used in SCR power controllers as well as SCRs.

Diode The diode is a semiconductor device that allows current to flow in only one direction. Electric current flows through a diode ONLY when the anode is positive and the cathode is negative (Figure 6a). If the anode becomes negative (and cathode positive), the diode shuts off. Figure 6

+ Anode

- Cathode Current Flow

a. Diagram of a diode

V

+ 0 _

1st cycle

2nd cycle

3rd cycle

b. Alternating Current (AC) through a Diode

Thus, when AC flows through the diode, only half of the wave can get through (Figure 6b). Why? AC alternates its polarity every half cycle. Whenever AC is in a negative alternation, the diode shuts off and doesn’t let any current through! This is a problem, since the heater will only receive 1/2 of the available current. Also, we can’t switch the diode ON and OFF to control current flow to the heater. Is there something better? Yes! We can use a SCR.

SCR - A Modified Diode

The SCR is a modified diode (Figure 7a). The diode now has a “gate.” To turn v the SCR ON, the anode must be positive with respect to the cathode, load current + must be available (complete circuit) and the gate must be made positive with 0 respect to the anode. The SCR will then stay energized (in conduction) until the anode voltage is removed or the anode/cathode polarity 1st reverses. 2nd 3rd cycle

cycle

cycle

Removing the gate signal while the SCR is conducting, will not de-energize the SCR. 6

Watlow Educational Series SCR - A Modified Diode (con’t)

Thus, when a SCR is conducting AC, the gate must be fired at the beginning of each cycle for current to flow (Figure 7b). If the gate is not fired, no current is conducted by the SCR, even if the anode and cathode are the correct polarity. So a SCR’s gate allows us to control current flow through the SCR. We did not have this ability to control current flow with a diode. Figure 7 Gate

+ Anode

- Cathode

V

Current Flow

a. Diagram of a SCR

+ 0 _

1st cycle

2nd cycle

3rd cycle

b. Alternating Current through a SCR

Unfortunately, the SCR (like the diode) only allows current to flow in one direction. Since AC switches polarity every half cycle, only half of the available current in AC is conducted through the SCR. What can we do to get current flow in both directions? Simple. Two SCR’s are simply placed back-to-back in parallel (Figure 8a). When the current is positive on the right side of the SCRs, SCR #1’s gate is fired and conducts current. When AC changes polarity, SCR #2’s gate fires and SCR #2 conducts current in the other direction. The result is that the full AC sine wave is sent to the heater, allowing us to use the full power available (Figure 8b). This v cycle continues then until the gates are no longer fired. Figure 8

SCR #1 +

-

-

+

V SCR #2

a. Diagram of a Pair of Back-to-Back SCRs

+ 0 -

1st cycle

2nd cycle

3rd cycle

+ 0 _ b. Alternating Current through Back-to-Back SCRs

Triac You may have heard of a triac before. A triac is simply a modified set of back-toback SCRs. Instead of having two gates, there is only one (common) gate (Figure 9). This saves on space. Triacs are typically not used for voltages over 240 volts v or large currents. Why? A triac generates more heat than a SCR pair due to the + higher watt density of the triac, thus making it less efficient. The triac conducts 0 SCRs alternate the load on two separate current full time where the back-to-back silicon chips. Triacs also typically have lower blocking voltage ratings than SCRs.

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SCR Power Control Triac (con’t) Figure 9 Diagram of a Triac Common Gate

Common Gate

We wanted to mention the triac, as you may hear of it from time to time. In fact, some Watlow temperature controllers still use a triac as an output device to switch small load currents. We have learned about SCR’s, but still don’t know much about how SCR’s are switched ON and OFF. We explore this next.

SCR Firing Methods Before we explore SCR firing methods, let’s take a small step back and review what electrical noise and zero-cross switching are. This will help us better understand advantages and disadvantages of the various SCR firing methods.

Electrical Noise Electrical noise (also known as radio frequency interference or RFI) is an electrical impulse which travels along any electrically conductive medium or is radiated through the air. Sensitive electronic equipment using microprocessors and integrated circuits may not function properly when they are exposed to electrical noise. Why? It interferes with their ability to operate properly. The higher the voltage switched, the more electrical noise is generated. This explains why the static you hear on an AM radio is vastly greater from a lightning strike than a light switch turning on and off!

Zero Cross Switching

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What are sources of electrical noise? Any electrical device which produces rapid or large changes in voltage potential will generate electrical noise. Examples are switches, relays, inductive loads (like motors, coils, solenoids), phase angle fired SCRs, electrical welding machinery, fluorescent and neon lights, etc. You can see and hear electrical noise when listening to an AM radio station. When a light switch is switched or an electric motor is operated near the radio, you hear “static” from the speakers. That static is really electrical noise picked up by the radio and converted into sound. The electrical noise interferes with the radio’s ability to process and output a clear AM radio signal. You can just imagine what this “noise” does to sensitive electronic equipment.

How do we eliminate electrical noise? Good question. At what point is the SCR’s gate normally fired? Look at the AC waveforms through the SCR in Figures 6 to 8. What do you see? The gate is always fired (switched ON) at zero voltage. Why? To eliminate (or at least greatly reduce) electrical noise, we want to switch the SCR ON and OFF at the lowest possible voltage. See Figure 10. Switching the SCR ON when the AC sine wave crosses zero voltage is called zero-cross switching.

Watlow Educational Series Zero Cross Switching (con’t) Figure 10

V

+ 0 _

SCR SWITCH-ON POINTS SCR Switch-on-Points

All SCRs exhibit a forward voltage drop while in conduction of about 1 to 1.25 volts. This “forward voltage drop” produces a very small amount of electrical noise and generates some heat. How do we get the heat away from the SCR? We use a heat sink to dissipate the heat from the SCR. All SCR power controls use heat sinks if they are switching a significant load current.

Contactor Firing

A SCR power control can be used as a contactor. Contactor is just another word for a relay. Typically, it is only used as a contactor if it is switching very high amperage loads (usually over 75 to 100 amps). The control output signal required by the SCR is some type of ON/OFF signal, like a switched DC signal. Thus, the SCR can function just like a large relay or SSR. Of course the SCR is zero-cross switched to minimize electrical noise. Using SCRs as contactors to control heaters is not recommended at longer time bases. However, when using a temperature controller with a switched VDC output on a fast time base (4 seconds or less) can make for a simple and effective temperature and power controller combination.

Burst Firing Burst firing SCR power controllers provide short “bursts” of load current to the heater. These short bursts of current provide just the right amount of power requested by the temperature control output. How many and how often these “bursts” of current occur depends on the time base of the power controller. Time base is basically the “cycle time” programmed into the SCR power controller instead of being set in the temperature controller. However, the time base is often not user selectable.

The time base is basically the same thing as the cycle time within a PID temperature controller. With SCR power controllers, however, the time base can be set by the power control itself. The burst firing SCR power controller can use two types of time bases: fixed and variable. Fixed Time Base A fixed time base means that the “cycle time” on the SCR power controller is permanently set or fixed (not adjustable). During this time period (1 second, for example), the SCR power controller will turn the SCR ON and OFF to provide the correct percentage of power to the heater. Let’s work through some examples to clarify this.

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SCR Power Control Burst Firing (con’t) Example: A temperature controller’s process output (4-20mA signal) commands the SCR power controller to provide 40% power. The SCR has a fixed time base of 1 second. The power supply is 60 hertz (60 cycles per second) AC. How is the SCR burst fired to provide 40% power to the heater during this 1 second time period? The SCR power controller uses the percent power requirement (40%) to calculate how many AC cycles should be allowed through the SCR during 1 second. The result (shown in Figure 11a) is that the first 24 cycles (40% of 60 cycles) will be allowed through the SCR. The SCR is then turned off for the other 36 cycles (60% of 60 cycles). Example: If the process output demands a 90% power requirement, how many cycles will the SCR allow through (time base = 1 second)? The SCR is fired enough to allow 54 cycles (90% of 60 cycles) through (Figure 11b). After 54 cycles, the SCR is not fired for the remaining 6 cycles (10% of 60 cycles). Figure 11

24 Cycles ON

36 Cycles OFF

a. Burst Firing with Fixed Time Base - 40% Power Requirement

54 Cycles ON

6 Cycles OFF b. Burst Firing with Fixed Time Base - 90% Power Requirement

See how this works? Notice that we are still switching the heater ON and OFF. We are just switching it ON and OFF (or cycling) within the 1 second time base. As the power requirement changes, the number of AC cycles allowed through the SCR per second also changes. The load current flowing through the heater is only continuous for a full ON condition. Therefore, it is difficult to measure load current with a meter when controlling at a short time base.

Exercise One A burst firing SCR is used with a 1 second time base. If 20% power is required, how many ON cycles and OFF cycles are there each second. A 60 Hz power supply is used. Draw a diagram to illustrate this. Answer on page 25.

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Watlow Educational Series Variable Time Base A SCR power controller with a variable time base changes the time base according to the power requirement. Burst firing with a variable time base uses the smallest possible number of AC cycles to deliver the required percentage power to the heater. Again let’s do 2 examples and compare these to the fixed time base examples on the previous page. Example: A temperature controller’s process output commands the SCR power controller to provide 40% power. The SCR has a built in variable time base. The power supply is 60 hertz (60 cycles per second) AC. How is the SCR burst fired variable time base to provide 40% power to the heater during a 1 second time period? First, the variable time base SCR calculates the smallest possible number of cycles to produce 40% power. For 40% power, 2 AC cycles out of every 5 cycles (2⁄5 = 40%) are allowed through the SCR (Figure 12a). Compare this to a fixed time base of one second at 40% power - 24 cycles are ON and 36 cycles are OFF. Example: If the process output demands a 90% power requirement, how many cycles will the SCR allow through using a variable time base? Now the SCR will be ON 9 out of every 10 cycles (9/10 = 90%). Notice how the time base changed from 5 cycles in Figure 12a to 10 cycles in Figure 12b! Now you know how variable time base got its name? The time base changes automatically to deliver the shortest possible bursts of power to the heater. Figure 12

V

+ 0 _ 2 Cycles ON

3 Cycles OFF

a. Burst Firing with Variable Time Base - 40% Power Requirement

V

+ 0 _ 9 Cycles ON

1 Cycle OFF

b. Burst Firing with Variable Time Base - 90% Power Requirement

What is the main advantage of variable time base over fixed time base? Because variable time base typically switches the heater ON and OFF more often, heater life is greatly increased as well as the SCR.

Why is variable time base preferred over a fixed time base? As you can see from our analysis above, the ON/OFF switching of the heater happens much more often with variable time base. We know that the more often the heater is switched, the less temperature variations the resistance element has. The nearly constant load current to the heater keeps the heater’s resistance element temperature nearly constant and therefore less thermal fatigue. This provides a longer heater life.

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SCR Power Control Variable Time Base (con’t) Contrary to the definition a solid state device (SCR in this case) is not totally solid state. There are mechanical connections that can fatigue just like a heater element. Therefore variable time base also extends the life of an SCR.

Exercise Two A burst firing SCR uses a variable time base. If 20% power is required, how many ON cycles and OFF cycles will provide 20% power to the heater? Draw a diagram to illustrate this. Compare this to Exercise One. Answer on page 25.

Phase-Angle Firing

In this method of SCR firing, the proportioning action takes place every consecutive half cycle in the AC sine wave. The time base then, is equal to one half AC cycle! This is fast! The firing point is variable within this half cycle time period to achieve a very accurate proportional control of electric current through the SCR. If the SCR gate is fired early in the half cycle, the power output of the heater is high. Why? If most of the current gets through the SCR, the heater will produce lots of power. If the gate is fired late in the half cycle, only a small increment of power passes through the SCR. Then the amount of power (heat energy) produced by the heater will be very little. If we again use the 40% and 90% power requirement examples, you can see the affect of phase angle firing on the current flow through the SCR (Figure 13). Figure 13

+

V0 -

40% Power Requirement a. Phase Angle Firing - 40% Power Requirement

+

V0

+

- V0 -

40% Power Requirement

b. Phase Angle Firing - 90% Power Requirement

The point at which+ the SCR’s gate is fired in the AC sine wave is continuously variable across the V 0half cycle. Where it is fired depends on the power required by the heater. Current- flow to the heater is practically continuous! Thus, there is no temperature variation experienced by the resistance element in the heater. As a 12

V

+ 0 -

Watlow Educational Series Phase-Angle Firing (con’t) result, you may think that phase angle firing provides us the best resistance element control and the longest life possible. However, Watlow recommends zero cross variable time base for nichrome resistive elements. The reasons why are discussed later in this SCR study guide As you may have noticed, when the SCR is phase angle fired, it is NOT zero-cross switched! What problem will we have if it is not zero-cross switched? Phase angle fired SCRs generate a lot of electrical noise! The electrical noise appears as voltage “spikes” on the AC sine wave (Figure 14). These voltage spikes were left out of Figure 13 for clarity. Figure 14

+

V0 -

The “choppy” sine wave pattern in Figure 14 produces harmonic frequencies which add to the electrical noise in a circuit. Harmonic currents also add to the total load current in the circuit. The electrical system components must be sized to account for these extra harmonic currents. Sometimes this extra current is referred to as voltamps reactive (VARs)

SCR Review Questions

This is a good place to stop and review what we have discovered so far. Please take a few minutes to answer the following questions. Answers are on page 25. 1. A SCR power controller is like a relay, except that it can switch the heater ON and OFF much more rapidly. This increases heater life. True or false. 2. Name the 3 main parts of a SCR power controller. 3. Diodes and SCRs conduct current in both directions. True or false. 4. Why are back-to-back SCRs used?

5. Why do we prefer to use zero-cross switched SCRs?

6. Explain the difference between fixed and variable time base burst firing.

7. a.) Describe what phase angle firing is. b) In the left margin use diagrams to compare variable time base burst firing and phase angle firing. Compare them using a 60% power output on an AC 60 Hz power supply.

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SCR Power Control Power Control and Heater Life

As we learned in Book 2 (Thermal Systems and Electric Heaters), all resistance elements eventually burn out. We also know that we can greatly extend heater life by reducing the temperature cycling which the resistance element experiences. That is where the true advantage of the SCR power control lies - it greatly reduces the temperature cycling on the resistance wire. Well, how much does it reduce this temperature cycling? At this point, let’s summarize and compare what we know about the various types of output devices. The point we want to compare is cycle time (or time base) and what effect it has on resistance element temperature and hours to failure.

Electromechanical Relay

Mercury Displacement Relay (MDR)

Normally, this type of relay operates on a cycle time of 10 seconds or longer. When an ON/OFF cycle takes place over 10 to 30 seconds, the resistance element temperature experiences very wide temperature swings (Figure 15). Wide temperature swings accelerate resistance wire oxidation and burn out the heater much more quickly. The wire expansion and contraction work hardens (fatigues) the element and then it breaks.

Because the MDR uses liquid mercury to make contact and conduct electricity, the life of the MDR is much longer. Consequently, we can use a shorter cycle time. The typical cycle time for MDRs is 4 to 5 seconds. Notice in Figure 16 the dramatic reduction in resistance element temperature swings. This is a result of quicker switching (cycling every 5 versus every 30 seconds). The advantage of the MDR is not only in the longer life of the heater, but in the longer life of the mercury relay itself. Even when switching so quickly, the MDR will outlast an electromechanical relay. Figure 15 Time-Temperature Profile for an Electromechanical Relay

Figure 16 Time-Temperature Profile for a MDR

2000°F

1700°F

1600°F

1500°F

1300°F

Electromechanical Process Set Point: Overshoot: Droop: Internal Temperature

2000°F

14 1700°F

30 Second Cycle Time 1600°F 300°F (1900°F) 254°F (1346°F) 2100°F

Mercury Displacement Relay Process Set Point: Overshoot: Droop: Internal Temperature

5 Second Cycle Time 1600°F 50°F 30°F 1830°F

Watlow Educational Series Solid State Relay (SSR) Since a SSR is solid state, it does not fail due to the mechanical wear or arcing of the contacts cycling ON and OFF. It can operate on a cycle time of about 1 second. Again, because of the shorter cycle time, we see a further reduction in the temperature swings of the resistance element (Figure 17). These are the same temperature swings we expect to see if using a burst firing - fixed time base SCR (1 second time base).

Burst Firing SCR Power Control

A variable time base - burst fired SCR further reduces the time base to a few AC cycles. The switching time is now so short that the resistance element sees very little if any temperature swings (Figure 18). Variable time base switching will extend the life of resistance element heaters. There is still temperature cycling of the resistance element with very high watt density heaters. Figure 18 Time-Temperature Profile for a Burst Firing SCR (Variable Time Base)

Figure 17 Time-Temperature Profile for a SSR or SCR with 1 Second Time Base 2000°F

2000°F

1700°F

1700°F

1600°F

1600°F

1500°F

1500°F

1300°F

1300°F

Solid State Relay Process Set Point: Overshoot: Droop: Internal Temperature

Phase Angle Firing SCR Power Control

1 Second Time Base 1600°F 4°F 5°F 1730°F

SCR (with burst firing) Process Set Point: Overshoot: Droop: Internal Temperature

Minimum16.6 millisecond time base 1600°F 0 0 1720°F

The time base for phase angle firing is one half cycle of the AC sine wave. The heater switching is now so fast that there is NO temperature cycling of the resistance element (Figure 19)! Even high watt density heaters will not experience temperature cycling when phase angle firing is used. However, as good as phase 2000°F angle is, it still is not recommended over zero cross variable time base firing for nichrome element heaters. 1700°F Random turn on into the sine wave at 50% power will create significant harmonic heater current. 1600°F

Solid State Relay Process Set Point: Overshoot: Droop: Internal Temperature

Figure 19 Time-Temperature Profile for Phase Angle Fired SCR 1 Second Time Base 1500°F 1600°F 4°F 5°F 1300°F 1730°F

SCR (with burst firing) Process Set Point: Overshoot: Droop: Internal Temperature

SCR (with phase angle) Process Set Point: Overshoot: Droop: Internal Temperature

16.6 millisecond time base 1600°F 0 0 1720°F

8.3 to 10 millisecond time base 1600°F 0 0 1680°F

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SCR Power Control Burst Firing SCR Power Control (con’t)

An accelerated life test was done to determine which output device provides the longest life. The results in Figure 20 once again confirm what we knew...quicker ON/OFF switching leads to lower resistance element temperature swings. This, FIG.in20turn, greatly extends heater life! Figure 20 Heater Life Vs. Time Base Relationship

Heater Life Vs. Time Base Relationship SCR Burst Firing 8000 7000 6000

Life (Hours)

5000

SSR

4000 3000

MDR

500

ElectroMechanical Relay

400 300 200 100 0

0

33 ms

1

5

30

Cycle Time (Seconds)

Exercise Three A customer wants to order a Watlow microprocessor-based PID temperature controller. This controller is required because of the fast response and accuracy it offers. She is thinking of using electromechanical relays to switch the heater loads. Is this a good choice? Based on what we just discussed and what you know about output devices, what control output-output device combination would you recommend? Explain your answer. Answer on page 25.

16

Watlow Educational Series Power Control Selection

Only a SSR or a SCR power controller allow a customer to take full advantage of a proportioning (PID) temperature controller’s capabilities.

If SCR power controllers and PID temperature controllers are so great, WHY do customers still use thermostats and relays in many types of equipment? Think about it a minute.... Good question, hey? Let’s find a good answer! Most of the time, a customer uses the least expensive control system which will do the job required. So, if a thermostat controls temperature within +- 25˚F and the thermal system performs well, then a thermostat will be used. As more accurate temperature control is required, the customer must use either ON/OFF or PID controllers. These controllers are more expensive, but must be used to provide accurate temperature control. The mistake many customers make is to invest in an expensive PID temperature controller and then to use electromechanical relays to do the power switching! That defeats much of the purpose in using a PID temperature controller in the first place! A PID control can do things very fast...an electromechanical relay can only switch very slowly (due to long cycle time). It’s a mismatch. To make the best use of a PID control’s capabilities, you must use a SSR or SCR power controller to switch the heater (or cooling system) load. That is basically how the selection of the temperature and power controls are made. Let’s journey more specifically into application of SCR power controllers. There are 3 types of heating loads which SCRs switch: Heaters with stable resistance elements, heaters with variable resistance elements and transformercoupled heaters. We explore these types of heater control next.

Stable Resistance Heaters

All Watlow heaters use nichrome or other type of stable resistance element. Special designed heaters, however, may employ temperature dependent resistance elements.

These heaters have resistance elements which maintain a fairly constant resistance value. For example, nichrome wire elements typically experience about a 5%+ increase in resistance at operating temperature. The resistance value may increase 10%+ above the cold resistance if the temperature moves up toward nichrome’s operating limit (close to 2000˚F or 1200˚C). However, a 5 to 10% increase is not much. Other “stable” resistance elements increase somewhat more than 10% as temperature increases. The majority of Watlow heaters use nichrome or other stable resistance elements. When switching nichrome element heaters, a SSR with a 1/2 to 1 second cycle time setting is recommended. If higher amp switching or faster switching is required to get better heater life (or quicker control response), use a variable time base burst firing SCR power controller. Variable time base burst firing provides excellent control, very little electrical noise and provides very long heater life. The SCR Selection Chart (Figure 25 on page 21) shows this selection process more clearly. Notice that the chart does not recommend phase angle fired SCRs to control stable resistance heating loads! Variable time base burst firing works best in most nichrome element heater applications. When phase angle firing directly into a resistive heater there is only the heater element resistance to limit the current in the circuit. When conducting inside the AC sine wave, say 50% power demand, the SCR will energize from zero to peak voltage within a few micro seconds. The dv/dt and therefore the di/dt (load current rise with time) will cause the heater element to ring physically and thus reduce heater life. With zero cross the voltage and current rise and fall with the sine wave from zero to maximum. This control method is much easier on the heaters and power controllers.

17

SCR Power Control Variable Resistance Heaters

There are a few resistance element materials which show large resistance value changes with temperature. Examples are tungsten, molybdenum, silicon carbide and graphite element heaters. Tungsten elements are used in high temperature tungsten-quartz tubular heaters. Molybdenum, silicon carbide and graphite heaters are used fig. for21 high temperature ovens and furnace applications. The graph in Figure 21 shows the temperature dependent resistance values of tungsten and molybdenum. Figure 21 Temperature Dependent Resistance Values of Tungsten and Molybdenum 100

Specific Resistance microohm-cm

90 80 70

Tungsten

60 50 40

Molybdenum

30 20 10 0

400 800

1200 1600 2000 2400 2800

Temperature ˚C

3200

Why is this large resistance value change a problem? Let’s use Ohm’s Law to help us out. Ohm’s Law states that V = IR. The supply voltage to a heater is constant. Therefore, the only terms that can change are current (I) and resistance (R). If, for example, V = 230V and R = 23Ω at operating temperature, what is the current through the tungsten element? It is 10 amps (230V/23Ω). What happens when the heater is shut off? Per Figure 21, the resistance value of the tungsten drops to about 1/15th its value at operating temperature! So the cold resistance is about 1.53Ω. The high in-rush current on start up when using tungsten or molybdenum-based resistance elements will destroy a SCR almost instantly (unless using I2t fusing and soft start).

What happens when the heater is switched on again? Since the resistance element is cold, the current will be 15 greater at operating temperature (150 amps)! Why? Since the resistance dropped to 1/15th its “hot” value, the current must increase by 15 times to make the equation 230V = 150 amps x 1.53Ω valid. This large in-rush current can blow semiconductor fuses, damage SCRs and cause heaters to fail prematurely. How do we eliminate this problem? For these kinds of heaters, a phase angle firing SCR power controller is required. To prevent the SCR from burning up on heater start up, we use the soft start feature on a phase angle firing SCR. Figure 25 on page 21 shows you the selection process just described above.

18

Watlow Educational Series Variable Resistance Heaters (con’t)

Phase Angle Soft Start Soft start only operates during heater start up. When the signal is given to start the heater, the temperature controller tells the SCR to switch full ON. However, we know that if this happens, the SCR could be damaged. So in this start up condition, the soft start feature allows only a small amount of current through to the heater. As time goes on, the current is gradually increased until full power is finally applied to the heater (Figure 22). Soft start usually lasts for about 5 to 10 seconds, depending on the SCR power controller used. Some power controllers allow this soft start time to be adjustable by the operator. Figure 22 Soft Start Illustrated

V

+ 0 _ Time

Soft start essentially allows the resistance element to preheat or warm up. This increases element resistance before the full load voltage is applied through the SCR to the heater. This ensures that the SCR is not harmed and that the heater will have a very long life. Soft start is usually adequate for the tungsten quartz lamp heaters because the warm up period is so short, usually only a few AC cycles. Phase Angle With Current Limiting Soft start gradually increases the current flow and thus the power supplied to the heater during initial heater start up. It is still possible, however, that after the soft start (or at some other time during operation) too much current may pass through the SCR. This, as we know, can destroy the SCR and the heater. To prevent this from happening, a current sensing transformer is built into the SCR power controller. The transformer senses the current flow through the SCR. If the load current is higher than a preset current limit, the power controller electronics reduce the current to the maximum limit value (Figure 23). When the load current through the SCR drops back below the maximum current limit, then the full power of the AC sine wave is allowed through. Figure 23 Current Limiting in Action

+

V0 _

Soft Start

Current Limiting

19

SCR Power Control Variable Resistance Heaters (con’t)

Transformer-Coupled Heating Loads

Low voltage Watlow ceramic fiber heaters are often transformer-coupled.

Current limiting cannot help, though, when a short circuit occurs. When a short circuit occurs there is almost no resistance in the circuit. The rise in current is so swift that it destroys the SCR. The only way to prevent a short circuit from destroying the SCR is by using a semiconductor fuse. This fuse will blow within 1 ⁄2 cycle of the short circuit condition. ALWAYS use semiconductor fuses whenever SCR power controls are used! It is a small price to pay to protect the customers investment! Semiconductor fuses have a guaranteed specified I2t value where the fuse will blow if the short circuit current exceeds that spec. The I2t of the fuse should always be sized below the I2t specification of the SCR. Normally the fuse is selected by the power controller manufacturer.

Heaters used in very high temperature applications tend to have low voltage ratings (say, under 100V). Low voltage ratings are required, because the heaters typically have very low resistance values. Using a lower voltage provides the proper amount of current required to power the heaters. A transformer is used to step down the high voltage power supply to the low voltage required by the heater. A phase angle fired SCR is again required to control current flow through the transformer (and thus the heater). The transformer and phase angle fired SCR are usually connected as shown in Figure 24. Notice how the SCR power controller is connected on the HIGH VOLTAGE side of the circuit. It is much less expensive to control high voltage and low current than the other way around! It is important that the transformer be sized, voltage and KVA, properly to ensure a strong power supply. The voltage should be selected so that the power controller is using as much of the full sine wave as possible (phase forward) to reduce the harmonic currents and improve power factor. Figure 24 Typical Connection Scheme for Transformer-Coupled Heaters SCR Power Controller

L1 High Voltage Supply

Low Voltage Side

L2 Transformer

Heater

A phase angle SCR is required, because it has a built-in circuit which ensures that the transformer always receives alternating plus and minus AC voltage. If, for some reason, the transformer received two pulses of the same polarity, it could overheat the transformer and blow the semiconductor fuse. The SCR Selection Chart in Figure 25 again shows this for transformer-coupled heating loads.

SCR Selection Chart

20

To simplify our selection of the proper SCR power controller, we can use the flow chart shown in Figure 25. First, we determine the type and resistance characteristic of the load. Then, we find the recommended type of SCR firing method. Finally, we find the type of temperature controller output required to command the SCR power controller. Example: You have quoted a Watlow radiant panel heater to a customer. What type of SCR power control can you recommend?

Watlow Educational Series SCR Selection Chart (con’t)

From Figure 25, we see that Watlow heaters are stable resistance loads. Thus, we can choose a solid state contactor (SSR) or a burst firing SCR power controller. The temperature controller output required for a SSR is a time proportioning output (like switched DC). The temperature controller output required for a burst firing SCR power controller is a process output (like 4-20mA). Figure 25 SCR Selection Chart

Characteristics of the Load

Notes: *1. Nichrome heater elements change resistance less than two times in their operating temperature range. *2. Heaters that change resistance include: • Tungsten increases over 16 times from cold to hot • Silicon carbide changes with temperature and age • Molybdenum and graphite increase resistance with the temperature and are often used on the secondary of a transformer *3. Transformers can become DC saturated if two pulses of the same polarity are applied in sequence which can cause overheating and high currents that will damage the SCR. Burst firing should not be applied.

Typical Load

SCR Firing Method Required

Stable *1 Resistance

Resistance *2 Change

Inductance *3

Nichrome Cartridge Circulation Strip Tubular Mica Strip Quartz Radiant

Tungsten Quartz Silicon Carbide Glo Bars Molybdenum Graphite

Transformer

Burst Firing

Phase Angle

Phase Angle

Process (Analog)

Process (Analog)

Process (Analog)

Solid State Contactor

Temperature Control Time Output Required Proportioning

Exercise Four A customer wants to use a Watlow tubular immersion heater. The heater is rated for 480VAC, 3 phase, 50kW. He wants to use a thermostat connected up to an electromechanical relay to control thermal system temperature. The customer also requires accurate temperature control and fast response to temperature changes. Based on the heater and customer requirements, recommend the best temperature controller (thermostat, ON/OFF or PID), control output and switching device to meet customer requirements. Use Figure 25 if necessary. Answers on page 25.

21

SCR Power Control 3 Phase SCR Power Control It is actually quite simple to apply SCR power controllers to single and three phase powered heaters. Single phase heaters are connected as shown in Figure 26. Notice that only one set of back-to-back SCRs is used to control current to the heater. Of course, proper fusing must be used! Figure 26 Single Phase SCR Power Control L1

L2

Contact a Watlow sales agent or Watlow Controls factory for more detailed information on switching heaters with SCR power controllers.

There are many types of 3 phase connections for SCR power controllers. Each is dependent on the type of SCR firing required. Figure 27 illustrates that only 2 sets of back-to-back SCRs are required to control a burst fired, 3-phase heater. This is the least expensive way to control burst fired, 3-phase heaters. Remember that burst firing means that the SCRs are zero-cross switched. The key advantages of a 3-phase, two leg control over a 3-phase, 3-leg are: best zero cross, less switching noise, lower cost, less power dissipation, simpler design, and a physically smaller package. In operation the L2 phase is the return path for the switched L1 and L3 phases. Figure 27 Three Phase-Two Leg SCR Power Control (Burst Firing Only) L1

L2

L3

When phase angle fired power control is required, one common configuration is a hybrid 3 pair SCR/Diode combination (Figure 28). This may be less expensive than 3 pairs of back-to-back SCRs and generates less heat. This hybrid design can be used for burst fired control of 3ph heaters, but is normally not recommended. The diodes are the return path for the switched SCRs, this power control configuration is slowly going away since SCR costs are more comparable and advanced technology is simplifying SCR gating. Figure 28 Three Phase-Three Leg Hybrid SCR Power Control (Phase Angle Firing Only) L1

L2

L3

22

Watlow Educational Series 3 Phase SCR Power Control (con’t)

The most common configuration for phase angle fired 3-phase heaters, is to use three pairs of back-to-back SCRs (Figure 29). This control scheme must be used when there is an unbalanced load (resistances of various legs are different). It also must be used for delta-to-delta 3ph transformer control. Figure 29 Three Phase-Three Leg SCR Power Control (Phase Angle Firing Only) L1

L2

L3

Finally, when a 3-phase, 4-wire (grounded wye) heater must be controlled, 3 pairs of back-to-back SCRs are again used with an uncontrolled neutral (Figure 30). This is for burst fired control applications. In operation the SCR switches from phase to neutral. Figure 30 Three Phase-Four Wire SCR Power Control (Burst Firing Only) L1

L2

L3

Neutral

Booklet Review Questions

We’ve come a long way on our journey through SCR power control! Now, let’s review by putting into practice what you have so diligently studied. Answer the following questions. If you can’t answer a question, go back into the book and review that section. Answers to all questions are on page 26. 1. Why do we use a SCR power controller in a thermal system? a) To increase work load life. b) To increase electromechanical relay life. c) To increase heater life. d) To increase the ability of a PID temperature controller to respond to temperature changes in the work load and increase heater life. e) Both a and c are correct. 2.

Briefly explain why SCR's are placed back-to-back in parallel.

23

SCR Power Control Booklet Review Questions (con’t)

3. What does "time base" mean? a. It is similar to cycle time on a PID temperature controller. b. The number of cycles per second (hertz). c. The time required for the work load to reach set point. d. The time during which a SCR power controller cycles the heater ON and OFF. e. Only a and d are correct. f. Only a and c are correct. 4.

A PID temperature controller has a 0-5VDC process output. The process signal is fed to a burst firing SCR power controller (with a variable time base). The power supply has 230VAC, 60Hz. The process output signal is 3.5 volts. What is the current flow through the SCR to the heater? a. 42 cycles ON, 18 cycles OFF. b. 7 cycles ON, 10 cycles OFF. c. 7 cycles ON, 3 cycles OFF. d. 3 cycles ON, 1 cycle OFF.

5.

Which of the answers in question #4 above is correct if the SCR power controller has a fixed time base of 1 second?

6.

A quartz tubular heater uses tungsten resistance elements. What method of SCR firing and options should be chosen for this heater? a. Burst firing, fixed time base, current limiting. b. Burst firing, variable time base. c. Phase angle firing, soft start. d. Both a and b are correct.

7. A set of Watlow Multicell heaters is used to heat gases to very high temperatures. The heaters are rated 480VAC, 3ph, 20,000W. Which temperature control method, control output and output device would you choose to control these heaters. Please explain your answer.

8. Molybdenum disilicide heating elements are used in a high temperature furnace application. Which temperature control method, control output and output device would you choose to control these heaters. Please explain your answer.

9. A low voltage ceramic fiber heater is operated on 60VAC. A transformer is used to step down the voltage. What method of SCR power control do you recommend for this application? Please explain your answer. 24

Watlow Educational Series Answers to Exercises and Review Questions

Answers to Exercises Exercise 1: First we multiply 20% (0.2) by 60Hz to calculate the number of ON cycles per second. 60 x 0.2 = 12 cycles. Therefore, in one second, at 20% power, there are 12 ON cycles followed by 48 OFF cycles. See Figure 31a below. Exercise 2: With variable time base, we have to find the smallest combination of ON/OFF cycles which will provide 20% power. From Exercise One, we know that 20% power is 12 cycles ON, 48 cycles OFF. We then reduce this proportion to its smallest possible value. We can divide both numbers by 12. The result is 1 cycle ON/4 cycles OFF. This will provide 20% power to the heater. See Figure 31b below. Figure 31

V

+ 0 _ 12 Cycles ON

48 Cycles OFF

a. Fixed Time Base of 1 Second - Burst Firing Under 20% Power

V

+ 0 _

1 Cycle ON

4 Cycles OFF

1 Cycle ON

4 Cycles OFF

b. Variable Time Base - Burst Firing Under 20% Power

Exercise 3: In any case, electromechanical relays are not a good choice. This is especially true since fast response and accuracy are required. It is much better to use at least a SSR, or perhaps a SCR power controller. Assuming an external SSR is used, either a switched DC control output or internal SSR must be used to switch the external SSR. Use a fast cycle time (about 1 second). If a SCR power controller is used, use a process control output and a variable time base burst firing SCR. Exercise 4: DO NOT USE THE THERMOSTAT! Use a PID temperature controller with a process output (like 4-20 mA) and a variable time base burst firing SCR power controller Answers to SCR Review Questions - Page 13 1. True. A SCR power controller switches power ON and OFF to the heater in much the same way as a relay. The faster switching increases heater life. 2. The SCR, control electronics and heat sink. 25

SCR Power Control Answers to Exercises and Review Questions (con’t)

3. False. They conduct current in only one direction. 4. This allows current to flow in both directions. One SCR conducts current in one direction, the other SCR conducts current in the other direction. 5. This greatly reduces electrical noise, because SCR switching takes place at or near zero volts. 6. In fixed time base, ON and OFF switching of the SCR must always take place during a fixed time period (like 1 second). Variable time base continually varies the time period (or number of cycles) to provide the quickest switching possible and the least amount of change on the heater. 7. a) Phase angle firing fires the SCR each consecutive half cycle. The current flow through the SCR is varied by firing the SCR either earlier or later in that half cycle. b) The smallest possible number of cycles required to get a 60% power flow is 3 cycles ON out of 5 cycles (3/5 = 60%). Thus the total time base is 5 cycles. Figure 32 below illustrates this. Figure 32

V

+ 0 _

3

Cycles ON

2

Cycles OFF

5 Cycles Time Base

Answers to Booklet Review Questions - Page 23 1. d) To increase the ability of a PID temperature controller to respond to temperature changes in the work load and increase heater life. 2. They are placed back-to-back in parallel so that AC current can flow both ways through the SCR pair. One SCR is used for current in one direction, one SCR is used for current in the other direction. 3. e) Only a and d are correct. 4. c) 7 cycles ON, 3 cycles OFF 5. a) 42 cycles ON, 18 cycles OFF. 6. c) Phase angle firing, soft start. 7. Use a PID temperature controller, a process output (like 4-20mA or 0-5VDC), and a burst firing, variable time base SCR power controller. Use the 3 phase, 2 back-to-back SCR controller shown in Figure 27. This temperature and power control system will provide the best life and temperature control.

26

Watlow Educational Series Answers to Exercises and Review Questions (con’t)

8. Molybdenum is (per Figures 21 and 25), one of the resistance elements that has a high change in resistance due to temperature. Therefore, a PID temperature control, process control output and phase angle fired SCR power controller are required. The phase angle control should have soft start and current limiting. 9. Transformers (per Figure 25) require phase angle fired SCR power controllers.

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